(1) Field of the Invention
The present invention relates to sensors which exploit a change in phase of an interrogation signal to determine a sensed parameter, and particularly, but not exclusively to fibre optic interferometric sensing. The present invention finds particular application in the filed of seismic surveying and imaging.
(2) Description of the Art
Fibre optic sensors employ a length of optic fibre arranged in such a way that a sensed parameter causes a strain to be imposed on the fibre. Typically the fibre is arranged in a coil, although other arrangements are possible. Such strain causes a change in phase of the optical signal propagating in that fibre, which change can be detected by interferometric techniques. A variety of different arrangements for this type of transducer have previously been proposed, many of which have the coil of optic fibre wound on a deformable core or mandrel, which undergoes radial expansion or contraction in response to the sensed parameter, such as sensed vibration.
Such fibre optic sensors can exhibit extremely high sensitivities, and have the advantage of being completely passive, employing no power at the sensing transducer. Such sensors have also proved popular in applications where large arrays of sensors are required, on account of the relative ease with which they can be multiplexed.
An example of such an application is seismic surveying in the oil and gas exploration industry, where large time multiplexed arrays comprising hundreds or even thousands of vibration sensors and/or hydrophones can be used to sense reflections of an incident pulse from geological formations beneath the sea bed. Sampling such an array at regular periods provides 3D time lapsed data on existing or potential new reserves.
A problem experienced with this approach to sensing is that, for a given sampling rate, signals above a certain amplitude threshold cause the phase based sensed information to become distorted, and can cause failure of the demodulation process. This effect, commonly referred to as overloading or overscaling is dependent on the frequency of the measured signal. In seismic systems this can cause a particular problem with the direct arrival of the incident pulse, especially when that pulse has been generated close to the sensors (usually by an airgun towed from a surface vessel as it passes over the array). It is desirable to be able to record this incident pulse without the distortion that overscaling can produce.
It is known in the field of optical metrology that a combination of two wavelengths can be used to measure relatively large optical path lengths, of the order of 1 mm for example, to extremely high accuracies using interferometric techniques. This has the effect that the light propagating through the interferometer can be considered as having the synthetic wavelength, giving rise to a synthetic phase or phase change. See for example R. Dandliker, R. Thalmann, and D. Prongue, “Two-wavelength laser interferometry using superheterodyne detection,” Opt. Lett. 13, 339-(1988), which describes a free space interferometer capable of operating at a synthetic wavelength created by two laser sources operating in multiple polarisation states.